This invention relates to a method for braking an alternating-current motor when the alternating-current motor is supplied by a frequency converter comprising an intermediate circuit.
When an alternating-current motor supplied by a frequency converter comprising an intermediate circuit is being braked, the voltage of its intermediate circuit tends to rise. This is due to the fact that the motor then acts as a generator, supplying the energy of its mechanical system to the intermediate circuit. The braking ability of the system is thus also limited by the permitted increase in voltage for the intermediate circuit. If the voltage rises too much, the braking torque will have to be limited or, alternatively, excess energy will have to be fed into the network by means of a suitable arrangement such as an inverter. Excess energy can also be fed into a thermal resistance, whereby said resistance and a separate braking chopper are required. However, these alternatives require the use of additional devices, which complicate the system and add to the costs involved. The third alternative to boost braking is to employ direct-current braking; this, however, causes the control to lose the knowledge of the rotation speed of the motor. Consequently, direct-current braking is only applicable when the motor is to be braked to a complete standstill.